The invention relates to a selector stage. The invention also relates to a selector and a selector system comprising a plurality of selector stages.
Such a selector is disclosed in U.S. Pat. No. 638,249. The selector is a step-by-step driven switch which can move in 10 steps in the vertical direction and can rotate also in 10 steps in the horizontal direction and establish a connection between the input and one contact of a contact bank having 10.times.10 contacts. This selector is usually designated as a two-motion switch. The first auomatic telephone exchanges were equipped with this type of selector and were called Strowger exchanges, after the inventor of the selector. Use of these selectors made it possible at that time to carry on telephone calls without an intermediary operator. The advantage of these exchanges remains the fact that a decentralized structure obtains a relatively high reliability. Disturbances remain limited as much as possible to their immediate environment ("graceful degradation"). A disadvantage is that the control portion is integrated with the speech path portion which resulted in a low degree of flexibility, inter alia as regards the network structure and the control. Further developments show that by adding a register to the selector a first step was made towards centralization of the control: namely, the register is used for a brief period of time (for the period of time needed to establish the connection) and the register can thereafter be employed for one or several other selectors.
Further centralization was accomplished by means of the exchanges commonly referred to as cross-bar system exchanges in which centralized adjustment elements ("markers") are provided which change the dialling information and make it suitable for energizing the cross-bars. In contrast with the Strowger exchanges where the selectors are directly driven by the dialled numbers (direct control), in the cross-bar exchange the dialling information is first taken in by a marker before the selector is set ("indirect control").
A very high degree of centralization was achieved with the exchanges which are commonly referred to as stored program controlled exchanges. Virtually all the control is effected centrally, as a result of which the speech path network and the control form substantially distinct portions. These telephony networks have the drawback that they are very complex which is inter alia reflected by their capacity (number of lines), number of components, number of people involved in their design, design time and the substantially unlimited technical possibilities determined by the programs.